A vehicular safety system with a diaphragm-type pressure sensing apparatus is disclosed in JP-A-H2-249740. In the safety system, the pressure sensing apparatus is built into a side door of a vehicle and detects a shock wave of a collision. Thus, the safety system inflates an airbag when the collision occurs.
As shown in FIG. 2, this type of pressure sensing apparatus includes a pressure sensor 1, a circuit board 2, a housing 3, and a cover 4.
The pressure sensor 1 is constructed such that a semiconductor chip having a diaphragm is packaged in a resin-molded package with an opening through which the diaphragm is exposed.
The circuit board 2 has a mounting surface where the pressure sensor 1 and the circuit board 2 can be electrically connected. The circuit board 2 is fixed to the housing 3 and the pressure sensor 1 is mounted to the mounting surface of the circuit board 2. The circuit board 2 includes a signal processing circuit 21 for processing a detection signal output from the pressure sensor 1 and a constant voltage circuit for supplying a power voltage to a pressure transducer (e.g., a strain gauge sensor) mounted to the diaphragm of the pressure sensor 1. The housing 3 includes a pressure inlet port 6, a connector 8 having connector terminals 9, and a bottom wall 30. The connecter terminals 9 are soldered to the circuit board 2 inside the housing 3.
Pressure to be measured is introduced into a pressure sensing surface of one side of the diaphragm of the pressure sensor 1 through the pressure inlet port 6 of the housing 3. A vacuum chamber maintained at a reference pressure is located on the other side of the diaphragm and the diaphragm acts with reference to the reference pressure. When the measured pressure is introduced into the sensing surface of the diaphragm, the diaphragm is deflected toward or away from the vacuum chamber. The deflection of the diaphragm is converted into an electrical signal.
A perimeter portion of the sensing surface is pressed down on the bottom wall 30 of the housing 3 through a rubber-packing member 7 for sealing between the pressure sensor 1 and the bottom wall 30. Although this approach allows the measured pressure to be introduced into only the sensing surface of the pressure sensor 1 and prevents moisture ingress into the housing 3 through the pressure inlet port 6, undesired bending stress is applied to the pressure sensor 1 and the circuit board 2.
In order to reduce the bending stress, the pressure sensing apparatus is reinforced such that a resin-potting member 10 is provided on one side of the circuit board 2. However, in spite of the reinforcement, compressive stress in a thickness direction (i.e., top-bottom direction in FIG. 2) of the pressure sensing apparatus may be applied to the perimeter portion of the sensing surface of the pressure sensor 1 and a portion of the circuit board 2, the portion of which is positioned close to the perimeter portion of the sensing surface. The compressive stress affects circuit components such as the diaphragm of the pressure sensor 1, thus reducing accuracy of the pressure sensing apparatus. The rubber-packing member 7 increases the size, weight, and material cost of the pressure sensing apparatus.
In assembly of the pressure sensing apparatus, accurate alignment between the sensing surface of the diaphragm of the pressure sensor 1 and the pressure inlet port 6 is required. Further, the circuit board 2 is required to be fixed to the housing 3 while the pressure sensor 1 mounted to the circuit board 2 is prevented from tilting. The requirements make the assembly of the pressure sensing apparatus complicated.